21606640 has 80 divisors (see below), whose sum is σ = 56174976. Its totient is φ = 7660800.

The previous prime is 21606623. The next prime is 21606671. The reversal of 21606640 is 4660612.

It is a happy number.

It is a tau number, because it is divible by the number of its divisors (80).

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 37555 + ... + 38125.

Almost surely, 2^21606640 is an apocalyptic number.

21606640 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21606640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28087488).

21606640 is an abundant number, since it is smaller than the sum of its proper divisors (34568336).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21606640 is a wasteful number, since it uses less digits than its factorization.

21606640 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 638 (or 632 counting only the distinct ones).

The product of its (nonzero) digits is 1728, while the sum is 25.

The square root of 21606640 is about 4648.2943108198. The cubic root of 21606640 is about 278.5238642689.

The spelling of 21606640 in words is "twenty-one million, six hundred six thousand, six hundred forty".